Electrically ignitable primers have been previously used in military applications for high speed firing of various sized caliber ordnance, in blasting for mining operations, for automotive crash bag initiation and inflation, seismic guns, kiln guns, rocket motors, and pyrotechnic displays. However, many of these primers are not suitable for small arms such as rifles, pistols, and shotguns. Typically, electrically ignitable primers have been initiated by exploding bridge wires or hot wires in combination with semi-conductive mixture, pyrotechnic mix, or conductive mix. However, all of these electrical initiation systems suffer from relatively long ignition times. Both percussion and electrical primer compositions require expensive environmental handling procedures during both production and disposal. A primary concern is the amount of lead absorbed by humans from exposure to primer mix constituents, as well as the combustible by-products of lead-based primer compositions.
Primer mixes used in military ammunition must function reliably between the temperatures of −65° F. to +160° F. The reliability of current lead-free primer compounds degrade as temperatures approach −65° F. Attempts in improving the reliability of such primers has resulted in an increase in the hazards associated with their use in U.S. military weapons.
U.S. Pat. No. 5,717,159 issued on Feb. 10, 1998 to Dixon et al. teaches lead-free percussion primer mixes based on metastable interstitial (intermolecular) composite (MIC composition) technology. The lead-free percussion primer composition includes a mixture of about 45 weight % aluminum powders having an outer coating of aluminum oxide and molybdenum trioxide powder or a mixture of 50 weight % aluminum powders having an outer coating of aluminum oxide and polytetrafluoroethylene powder (Teflon®). The percussion primer mix is initiated by squeezing it between the base of the primer cup and an anvil fitted at the top of the cup by mechanical force. This action forces the metal (fuel) and metal-oxide (oxidizer) together with sufficient force to initiate a localized exothermic chemical reaction. Due to the very small particle size and level of compaction, the reaction propagates very quickly.
Initiation of a MIC material requires bringing the metal (i.e., the fuel, which in this case is the aluminum) and the metal oxide (i.e., the oxidizer—in this case MoO3) into close contact and in a quantity sufficient to sustain a reaction. Under normal conditions, this contact is prevented by the presence of an oxide film on the metal fuel. In the case of aluminum, the oxide adheres to the base metal with great tenacity and prevents oxygen from the oxidizer in reacting with the base metal even at elevated temperatures beyond the melting point. This method may work well with percussion cartridge primers; however, the method involved in the ignition of explosive materials operates quite differently in electrical initiation systems.
An electric igniter for artillery ammunition serves to ignite the primer charge of such ammunition. It typically includes a metal casing holding an initiator charge associated with an electric resistor. The resistor is electrically linked to a DC source and is further electrically linked to a contactor. Upon contact, the electric resistor is heated and initiator charge is ignited which further ignites the primer charge, usually via a booster charge. Although the approach just described works extremely well for explosive based cartridge primers, it is not applicable to MIC-based electrically initiated primers. Thus, because the metal particles that make up the powder have an oxide jacket, which is non-conductive, a MIC requires a different approach to initiate electrically.
From the foregoing, it will be appreciated that there is a need in the art for a lead-free electrical initiation system which is environmentally safe, provides primer mix that does not degrade as temperatures approached −65° F., and exhibits improved ignition times.